Ink-jet printheads operate by ejecting a droplet of ink through a nozzle and onto a recording medium, such as a sheet of paper. When a number of nozzles are arranged in a pattern, such as one or more linear arrays, the properly sequenced ejection of ink from each nozzle cause characters or other images to be printed on the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead has moved across the paper. The printhead is usually part of a disposable print cartridge containing a supply of ink, where the print cartridge is easily installed in and removed from the printer.
In one design of a thermal ink-jet print cartridge, the print cartridge includes: 1) an ink reservoir and ink channels to supply ink proximate to each of the nozzles; 2) a nozzle plate in which the nozzles are formed in a certain pattern; and 3) a substrate attached to a bottom surface of the nozzle plate, where a series of thin film heaters are formed on the substrate, generally one below each nozzle. Each heater includes a thin film resistor and appropriate current leads. To print a single dot of ink, an electrical current from an external power supply is passed through a selected heater. The heater is ohmically heated, in turn superheating a thin layer of the adjacent ink. This results in explosive vaporization of the ink causing a droplet of ink to be ejected through an associated nozzle onto the paper.
One example of this type of print cartridge is shown in FIG. 1 as print cartridge 10. Print cartridge 10 generally includes a body 12 which acts as an ink reservoir. Body 12 may have formed on it one or more projections, such as projection 13, to enable print cartridge 10 to be secured in place within an ink printer. The printhead portion 14 of print cartridge 10 includes a metal nozzle plate 16 (such as a gold-coated nickel plate), which has two parallel arrays of nozzles 17 formed in it using conventional photolithographic techniques. Nozzle plate 16 is attached by an adhesive to an underlying substrate (not shown) which includes heater resistors paired with each of the nozzles 17.
A flexible insulating tape 18 has formed on it a number of conductors which terminate in contact pads 20. The other ends of the conductors on tape 18 are connected, using tape automated bonding (TAB), to electrodes on the substrate.
When print cartridge 10 is properly installed in a moveable carriage of an ink-jet printer, pads 20 contact corresponding electrodes on the ink-jet printer which supply the energization signals to the various heater resistors on the substrate. When printing, the carriage scans print cartridge 10 across the width of a sheet of paper, and the paper is incrementally moved perpendicular to the direction of movement of print cartridge 10.
In a color printer, four separate print cartridges 10 are typically used and are carried by the same carriage across the sheet of paper. Typically, one of the four cartridges contains black ink, another contains cyan ink, another contains magenta ink, and another contains yellow ink.
FIG. 2 illustrates the pertinent portion of a color ink-jet printer with four print cartridges 24, 25, 26, and 27 secured within a single carriage 30. Carriage 30 is moved along stationary rod 31 back and forth across the paper sheet 32 in the direction shown by the arrow 34. A roller 35 shifts the position of paper sheet 32 as needed. In an actual embodiment, at least two spaced rollers are used to cause paper sheet 32 to be flat along where print cartridges 24-27 are scanned for printing.
Each of the print cartridges 24-27 may have nozzles 17 (FIG. 1) arranged so as to print 300 dots per inch (dpi) or more on paper sheet 32 along an axis perpendicular to arrow 34. This means that a nozzle 17 must be placed approximately every 3 mils along nozzle plate 16 in order to achieve 300 dpi.
In color printing, the various colored dots produced by each of the four print cartridges 24-27 in FIG. 2 are selectively overlapped to create crisp images composed of virtually any color of the visible spectrum. To create a single dot on paper sheet 32 having a color which requires a blend of two or more of the colors provided by print cartridges 24-27, the nozzle plates 16 on each of the cartridges 24-27 must be precisely aligned so that a dot ejected from a selected nozzle 17 in one cartridge overlaps a dot ejected from a corresponding nozzle 17 in another cartridge. This requires each of the nozzle plates 16 on print cartridges 24-27 to be aligned with respect to one another within a few tens of microns after being installed in carriage 30.
In the prior art, the print cartridge bodies 12 of FIG. 1 were made identical to one another so that, when multiple print cartridges 10 were installed in carriage 30, the print cartridge bodies 12 were all aligned with one another in carriage 30 irrespective of any misalignment of the nozzle plates 16 secured to the print cartridge bodies 12.
To align nozzle plate 16 on print cartridge 10 so that nozzle plates are positioned in ideally the same location on all the various print cartridges, nozzle plate 16 is typically glued in position on print cartridge 10 relative to a molded-in plastic datum formed on the print cartridge body 12 itself. This alignment process has a significant drawback in that the glue curing process causes nozzle plate 16 to slightly shift as the glue is being cured. In addition, molded-in stresses in plastic cartridge body 12 creep during the thermal curing process. Since this movement is substantially unpredictable, this alignment and gluing process can only produce print cartridges whose nozzle plates are positioned to an accuracy of .+-.35 microns.
Other, more expensive techniques have been used to achieve higher alignment precision. One of these techniques automatically detects any misalignment of the nozzle plates once the print cartridges have been installed in a carriage and then mechanically adjusts the positions of the print cartridges in the carriage. Using another relatively expensive method, an ink drop detector within the ink printer measures the location of a drop of ejected ink after being ejected from a nozzle, and a software algorithm compensates for any misalignment of the nozzle plates. Both of these techniques significantly increase the cost of the ink printer.
Thus, what is needed is an inexpensive and reliable method and structure for improving the alignment of nozzle plates (or other forms of nozzle members) on print cartridges installed in a carriage.